Imaging with extended depth of focus for use with polycromatic light

ABSTRACT

An imaging lens unit is presented, comprising an imaging lens having a lens region defining an effective aperture, and a phase coder. The phase coder may be incorporated with or located close to the lens region. The phase coder defines a surface relief along the lens region formed by at least three phase patterns extending along the lens region. Each of the phase patterns differently affecting light components of one of at least three different wavelength ranges while substantially not affecting propagation of light components of other of said at least three wavelength ranges. The surface relief affects light propagation through the lens region to extend a depth of focus for at least one of said at least three wavelength ranges.

FIELD OF THE INVENTION

This invention relates to imaging with extended depth of focus, and animaging lens unit configured accordingly, using polychromaticillumination.

BACKGROUND OF THE INVENTION

Extending the depth of focus of an imaging system is required forvarious applications, including medical application, such as ophthalmicapplications. Various techniques have been developed to extend the depthof focus of an imaging system.

For example, the earlier technique developed by the inventors of thepresent application provides an extended depth of focus of an imaginglens unit by applying a phase coding to the effective aperture of theimaging lens. The features of this technique are described in thefollowing patents and published patent applications: U.S. Pat. No.7,365,917; U.S. Pat. No. 7,061,693; US 2009/074239; US 2009/116096; U.S.Pat. No. 7,646,549, all assigned to the assignee of the presentapplication. Some other techniques for extending depth of focus forimaging and other optical applications are described for example in thefollowing patents: U.S. Pat. No. 6,554,424, U.S. Pat. No. 6,097,856 andU.S. Pat. No. 6,069,738.

Yet another technique is disclosed for example, U.S. Pat. No. 7,224,540,which describes an imaging system that has a wavelength dependent focalshift caused by longitudinal chromatic aberration in a lens assemblythat provides extended depth of field imaging due to focal shift andincreased resolution due to reduced lens system magnification. In use,multiple wavelengths of quasi-monochromatic illumination, from differentwavelength LEDs or the like, illuminate the target, either sequentially,or in parallel in conjunction with an imager with wavelength selective(colored) filters. Images are captured with different wavelengths ofillumination that have different focus positions, either sequentially orby processing the color planes of a color imager separately. Extendeddepth of field, plus high resolution is achieved. Additionally,information about the range to the target can be determined by analyzingthe degree of focus of the various colored images.

GENERAL DESCRIPTION

There is a need in the art for a novel imaging lens unit operable withpolychromatic illumination and configured to extend the depth of focuswhile maintaining high contrast imaging.

In order to enable imaging with the extended depth of focus the presentinvention provides a novel all-optical technique capable of obtaining anextended depth with desirably high-contrast by intentionally introducingappropriate chromatic aberrations in the imaging process. To this end,the invention provides a lens unit configured for imaging differentlight components of different wavelength ranges onto regions properlydisplaced with respect to one another along the optical axis, providingtogether an extended focal region, while an original lens used in thelens unit may be configured with chromatic aberration correction.

In imaging optics, chromatic aberration is a type of distortion thatoccurs when a lens focuses different wavelengths (different colors) oflight onto different spots in the image plane or onto different planes.This distortion is caused by different refraction effects of lens ontodifferent wavelength components of light propagating through the lens(i.e. different refractive indices of the lens matter for differentwavelength components of light), thus differently affecting thepropagation of light emerging from the lens.

Imaging systems are usually designed for use with a polychromaticwavelength range. To do so, an imaging system has to consider thechanges of the refractive index over different wavelengths. Thisrefractive index changes provide different optical powers of a lens fordifferent wavelengths of light and therefore create chromaticaberrations. For example, in a fused silica, crown glass or BK7 lenswith optical power of 1 diopter at green illumination the focal lengthfor red or blue illumination will differ by 1-2 centimeters, if nothingis done to correct chromatic aberrations. An imaging lens may be treatedto prevent chromatic aberrations by addition of a second lens to form alens unit with reduced aberrations, or by providing an appropriatecoating on the lens' surface.

The technique of the present invention utilizes phase coder extendingalong a lens region of an imaging lens (the imaging lens by itself mayor may not be treated to prevent chromatic aberrations). The phase coderdefines a patterned structure formed by at least three phase affectingpatterns each configured to affect the phase of light components of oneof at least three spectral ranges while substantially not affectingphase of light components of other spectral ranges. Additionally, one ormore of these phase patterns may be configured to provide focus shiftfor the corresponding spectral, range(s).

At least one of the at least three phase patterns is configured toprovide imaging with extended depth of focus for the correspondingspectral range. For example, any of such at least three phase patternsmay be configured as described in U.S. Pat. No. 7,365,917 assigned tothe assignee of the present application, or may be formed by opticalmasks and phase patterning as described in U.S. Pat. Nos. 6,554,424,6,097,856 or 6,069,738, all being incorporated herein by reference.

The phase pattern may be formed by a ring-like or grid-like arrangementof phase affecting regions, and is preferably configured to benon-diffractive.

Providing an imaging lens unit with capability to affect spectral rangesby different phase patterns respectively enables generation of a throughfocus MTF appropriately tailored for providing an extended depth offocus while maintaining high contrast imaging.

Preferably, the tailored through focus MTF is configured to provideproper axial displacement of the focal points for different wavelengthranges (chromatic channels) to provide imaging quality in which thespatial information of one channel is in focus and the information inthe other channels is relatively blurred. The proper axial displacementmay provide for large extension of the depth of focus and extension ofvisibility. For every axial distance, a different combination of one ormore in focus chromatic channels and one or more two defocused channelsmay be obtained.

The imaging lens unit of the present invention is specifically usefulfor various ophthalmic applications in which the brain adaptationprocess of a user can sharpen the information of the two defocusedchannels. Such adaptation process can be done since the visual spatialinformation received by the brain is based on a focused chromaticchannel. The imaging lens unit of the present invention may be used asspectacles lens, or for use in other ophthalmic applications, asintraocular lens, intracorneal lens or a contact lens.

The different phase patterns configured to affect light components ofspecific chromatic channels respectively while not affecting other lightcomponents may be such “color selective” by utilizing the periodicity ofelectromagnetic wave cycles.

Considering a phase pattern designed to affect light of a specificwavelength range, said phase pattern is designed such that it affectslight components of other wavelength ranges by addition of an integernumber of cycles (2πN phase difference) while affecting the specificdesired wavelength range by addition of a non-integer phase difference.

Thus, according to one broad aspect of the invention, there is providedan imaging lens unit comprising an imaging lens having a lens regiondefining an effective aperture, and a phase coder, said phase coderbeing incorporated with or located close to said lens region, said phasecoder defining a surface relief along the lens region formed by at leastthree phase patterns extending along said lens region, each phasepattern differently affecting light components of one of at least threedifferent wavelength ranges while substantially not affectingpropagation of light components of other of said at least threewavelength ranges, said surface relief affecting light propagationthrough the lens region to extend a depth of focus for at least one ofsaid at least three wavelength ranges.

Generally, the phase coder comprises a mask associated with a surface ofthe imaging lens within said lens region, where said mask has one of thefollowing configurations: (i) is integral with the lens region, in whichcase the surface relief is formed on at least one of the surfaces of thelens within said lens region; and (ii) is attached to the lens region,(iii) being spaced-apart from the lens region along an optical axis ofthe imaging lens.

The surface relief is a pattern formed by superposition of said at leastthree patterns. The surface relief pattern has features arranged alongsaid lens region and having predetermined dimensions along said lensregion and along an optical axis of the imaging lens. The surface reliefmay be configured to correspond to a predetermined height profile alongthe lens region, such that when polychromatic light passes through saidlens region, a depth of focus for at least one of said at least threewavelength ranges in said light is extended, e.g. depths of focus for atleast three wavelength ranges in said light are extended.

Preferably, said at least three wavelength ranges correspond to those ofprimary colors.

The phase pattern may be formed by at least one closed-loop zone of athickness different from that of its surrounding within the lens region.Such a closed-loop zone may have a ring-like geometry, or a polygonalgeometry. The phase pattern may be formed by at least two elongatedspaced-apart zones of a thickness different from that within a spacebetween the zones.

According to another broad aspect of the invention, there is provided animaging lens unit comprising an imaging lens having a lens region and anoptically transparent phase mask designed to provide at least threephase variation patterns to light components of at least three chromaticchannels respectively, the optically transparent mask having a heightprofile δd with respect to the lens region thereby providing said atleast three phase variation patterns, the height profile δd beingconfigured to satisfy the following condition:

$\begin{matrix}{{\delta \; {d(x)}} = {\frac{\lambda_{R}}{2{\pi \left( {n - 1} \right)}}\left( {{2\pi \; {N_{R}(x)}} + {\phi_{d}^{(R)}(x)}} \right)}} \\{= {\frac{\lambda_{G}}{2{\pi \left( {n - 1} \right)}}\left( {{2\pi \; {N_{G}(x)}} + {\phi_{d}^{(G)}(x)}} \right)}} \\{= {\frac{\lambda_{B}}{2{\pi \left( {n - 1} \right)}}\left( {{2\pi \; {N_{B}(x)}} + {\phi_{d}^{(B)}(x)}} \right)}}\end{matrix}$

wherein, λ_(R), λ_(G) and λ_(B) are wavelengths defining at least threechromatic channels, φ_(d) ^((R))(x), φ_(d) ^((G)) (x), φ_(d) ^((B)) (x)are said phase variation patterns, a is a refractive index of a materialof said optically transparent mask, N_(R), N_(G) and N_(B) are integernumbers, and x is a position vector along said lens region;

at least one of said phase variation patterns being configured toprovide imaging with extended depth of focus for the respectivechromatic channel.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 schematically illustrates and example of imaging lens unitaccording to the invention;

FIG. 2 exemplifies the phase coder suitable to be used in the imaginglens unit of the present invention; and

FIG. 3 shows an example of a through focus profile corresponding topolychromatic imaging achieved in the imaging lens unit of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is made to FIG. 1 illustrating an example of an imaging lensunit 100. The imaging lens unit includes an imaging lens 10 having alens region 12 and a phase coder 20 on top of the imaging lens. Theimaging lens unit may also include other lenses, apertures, a detectoror other optical element not specifically shown in this example.

The phase coder 20 is extending along the lens region 12 and configuredto provide phase encoding to light passing through the imaging lens 10of the imaging lens unit 100. The phase encoding may be integral withthe imaging lens 10, or it may be a phase mask integral with the lens10, attached to the lens or located close thereto. The phase coderdefines at least three phase patterns, each designed to affect lightcomponents of a specific wavelength range, to provide extended depth offocus to the lens system for the specific wavelength range while notaffecting light components of other wavelength ranges. It should e notedthat the imaging lens 10 may be treated to prevent chromatic aberrationsor not.

Reference is made to FIG. 2 showing a phase coder 20 presenting aso-called “combine” phase affecting pattern which is a superposition ofthree phase affecting patterns 21, 22 and 23, such that each of thephase affecting patterns is configured to affect light of differentspectral range while substantially not affect propagation of light ofother spectral ranges. The phase patterns 21-23, are configured as phaseencoding for providing different amounts of extended depth of focus andfocus shifts to the imaging lens unit. Such phase patterns or at leastone of them may be configured for example using the technique disclosedin U.S. Pat. No. 7,365,917 assigned to the assignee of the presentapplication. The phase patters are preferably configured to be nondiffractive. This can be achieved by forming a phase patterns from asmall number of phase affecting regions (i.e. low spatial frequency ofthe phase affecting regions), such that each of these regions has adimension much larger than a wavelength, for example, much larger than800 nm on either length or width of the region. The phase affectingpattern may be formed by one or more closed-loop phase affectingregions, e.g. a single ring-like region, or two or more spaced-apartconcentric ring-like regions. Alternatively, the phase affecting patternmay be formed by one or more polygonal regions (e.g. rectangle); or by agrid (e.g. spaced-apart lines). Generally, the phase affecting region isa region on the surface of the lens region having a thickness differentfrom that of its surroundings.

The different phase patterns are combined such that each of the patternsprovides phase variations to a specific wavelength range only, whilesubstantially not affecting light of different wavelength ranges. Thewavelength ranges may be chosen to define the primary colors used forimaging, which are typically defined by Red (630-740 nm), Green (520-570nm) and Blue (440-490 nm) colors.

As described above, the combined pattern 20 is configured to applydifferent phase encoding to light components of different colors(wavelength ranges). In this example, combined pattern 20 is asuperposition of patterns 21, 22, and 23 configured for phase encodingof respectively light components at wavelength around 700 nm (red),light components at wavelength around 550 nm (green) and lightcomponents at wavelength around 470 nm (blue).

The combined phase coder pattern may be in a form of surface reliefalong the lens surface. Such surface relief provides that segments alongthe surface of the lens 10 are configured to be at different heightsrelative to the lens region and along the optical axis of the lens.These segments creating a height profile along the lens region inducevariations to the optical path of light components passing through thelens region and therefore introduce phase variations to light output ofthe lens.

The combined phase pattern 20 can be described as a multi level profilesuch that every spectral range/wavelength of light passing through themulti level profile “experiences” a different phase profile. This ispossible due to the cyclic nature of wave and the fact that theeffective phase has periodicity of 2π. The height profile of the lenssurface preferably satisfies the following condition expressed byequation 1:

$\begin{matrix}\begin{matrix}{{\delta \; {d(x)}} = {\frac{\lambda_{R}}{2{\pi \left( {n - 1} \right)}}\left( {{2\pi \; {N_{R}(x)}} + {\phi_{d}^{(R)}(x)}} \right)}} \\{= {\frac{\lambda_{G}}{2{\pi \left( {n - 1} \right)}}\left( {{2\pi \; {N_{G}(x)}} + {\phi_{d}^{(G)}(x)}} \right)}} \\{= {\frac{\lambda_{B}}{2{\pi \left( {n - 1} \right)}}\left( {{2\pi \; {N_{B}(x)}} + {\phi_{d}^{(B)}(x)}} \right)}}\end{matrix} & \left( {{eqn}.\mspace{14mu} 1} \right)\end{matrix}$

where δd is the height profile along the surface of the lens; λ_(R),λ_(G), λ_(B) are wavelengths for the red, green and blue spectralchannels respectively; and φ_(d) ^((R))(x), φ_(d) ^((G))(x), φ_(d)^((B)(x) are the desired phage variations for the red, green and bluespectral channels respectively along the lens region (for example asshown is FIG. 2 by phase patterns 21, 22 and 23 respectively); N_(R)(x),N_(G)(x) and N_(B)(x) are integer numbers which typically vary from thered, green and blue (or violet) channels respectively and also alongpositions on the lens region. It should be noted that both, the surfaceprofile &i and the desired phase distribution/variation for eachspectral channel (φ_(d) ^((R)), φ_(d) ^((G)), φ_(d) ^((B))), arefunctions of location x along the surface of the lens. In this specificexample, the location x is described as a vector location along thesurface of the lens.

The height profile described by equation 1 is configured to providewavelength selective phase coding. Indeed, since the height differencesare typically of the order of a few tens of wavelengths, the heightdifferences may be of the order of a hundred wavelengths. Such largeheight variations provides flexibility in the phase variations byallowing selection of the integer numbers N_(R)(x), N_(G)(x) andN_(B)(x) from a large range of numbers.

Equation 1 describes a height profile which is a result ofcombination/superposition of three different phase patterns, while eachof these patterns effects a phase variation for the specific wavelengthrange (spectral channel) only. Since there is no exact analyticalsolution to equation 1, the design process may be done based onminimization of variations from equation 1, i.e. minimization of themean square errors ε_(R), ε_(G) and ε_(B) for the three spectralchannels. The minimization process allows calculation of the surfaceprofile according to the following set of equation 2:

$\begin{matrix}{{ɛ_{R} = {\int{{{{\frac{2\pi \left( {n - 1} \right)}{\lambda_{R}}\delta \; {d(x)}} - {2\pi \; N_{R}} - {\varphi_{d}^{(R)}(x)}}}^{2}{x}}}}{ɛ_{G} = {\int{{{{\frac{2\pi \left( {n - 1} \right)}{\lambda_{G}}\delta \; {d(x)}} - {2\pi \; N_{G}} - {\varphi_{d}^{(G)}(x)}}}^{2}{x}}}}{ɛ_{B} = {\int{{{{\frac{2\pi \left( {n - 1} \right)}{\lambda_{B}}\delta \; {d(x)}} - {2\pi \; N_{B}} - {\varphi_{d}^{(B)}(x)}}}^{2}{x}}}}} & \left( {{eqn}.\mspace{14mu} 2} \right)\end{matrix}$

Such minimization of the errors provides for the solution for N_(R),N_(G) and N_(B) and for the height, or surface, profile that willprovide the desired phase pattern.

The phase patterns affecting the different spectral ranges are designedto provide for at least some of the spectral channels an extended depthof focus and/or focus shift.

Reference is made to FIG. 3 exemplifying the focusing effect of animaging lens resulting from the above phase coding of the inventionapplied to the lens region, expressed in an imaging contrast for threespectral channels R, G and B, as a function of through focus shift(corresponding to the through focus MTF). It is clear from this figurethat the imaging lens focuses different colors onto different focalplanes with different depths of focus corresponding to a desiredcontrast pattern. The latter is such that the through focus MTF profilesfor the different colors are preferably overlapping such as to form anenvelope having an almost flat continuous region (or a plateau), aroundthe original focal plane of the lens (i.e. with no phase coder). Thephase coder is designed such that extended depth of focus achieved bythe phase coder prevents or at least significantly reduces the contrastreduction.

The lens unit of the invention with the appropriate phase coder ischaracterized by extended depth of focus, as compared to that of theoriginal lens, where the phase coder define a region along the opticalaxis of the lens and around or close to its original focal plane, whereat any point along said region at least one color is represented byin-focus image while the other colors may or may not be in focus.Considering imaging applications, the brain adaptation process by aviewer can sharpen the information of the defocused channels to receivethe visual spatial information through the focused channel and interpretthe entire image accordingly. Such an imaging lens unit may be used forvarious imaging applications including inter alfa ophthalmicapplications such as spectacles, and ophthalmic lenses (contact lenses,intraocular lenses, intracorneal lenses, etc.).

1. An imaging lens unit comprising an imaging lens having a lens regiondefining an effective aperture, and a phase coder, said phase coderbeing incorporated with or located close to said lens region, said phasecoder defining a surface relief along the lens region formed by at leastthree phase patterns extending along said lens region, each phasepattern differently affecting light components of one of at least threedifferent wavelength ranges while substantially not affectingpropagation of light components of other of said at least threewavelength ranges, said surface relief affecting light propagationthrough the lens region to extend a depth of focus for at least one ofsaid at least three wavelength ranges.
 2. The imaging lens unit of claim1, wherein said phase coder is integral with the imaging lens, saidsurface relief being that of at least one of surfaces of the lens withinsaid lens region.
 3. The imaging lens unit of claim 1, wherein saidphase coder comprises at least one mask attached to at least one surfaceof the imaging lens within said lens region.
 4. The imaging lens unit ofclaim 1, wherein said phase coder comprises a mask associated with asurface of the imaging lens within said lens region, said mask havingone of the following configurations: (i) being integral with the lensregion, (ii) being attached to the lens region, (iii) being spaced-apartfrom the lens region along an optical axis of the imaging lens.
 5. Theimaging lens unit of claim 1, wherein said surface relief is a patternformed by superposition of said at least three patterns.
 6. The imaginglens unit of claim 1, wherein said surface relief is pattern havingfeatures arranged along said lens region, said features havingpredetermined dimensions along said lens region and along an opticalaxis of the imaging lens.
 7. The imaging lens unit of claim 6, whereinsaid surface relief is configured to correspond to a predeterminedheight profile along the lens region, such that when polychromatic lightpasses through said lens region, a depth of focus for at least one ofsaid at least three wavelength ranges in said light is extended.
 8. Theimaging lens unit of claim 7, wherein said predetermined height profileis such that when polychromatic light passes through said lens region,depths of focus for at least three wavelength ranges in said light areextended.
 9. The imaging lens unit of claim 1, wherein said at leastthree wavelength ranges correspond to those of primary colors.
 10. Theimaging lens unit of claim 1, wherein the phase pattern is formed by atleast one closed-loop zone of a thickness different from that itssurrounding within the lens region.
 11. The imaging lens unit of claim10, wherein said at least one closed-loop zone has a ring-like geometry.12. The imaging lens unit of claim 10, wherein said at least oneclosed-loop zone has a polygonal geometry.
 13. The imaging lens unit ofclaim 1, wherein the phase pattern is formed by at least two elongatedspaced-apart zones of a thickness different from that within a spacebetween the zones.
 14. The imaging lens unit of claim 1, configured foruse in ophthalmic application.
 15. The imaging lens unit claim 14,configured for use as a contact lens.
 16. The imaging lens unit of claim14, configured for use as intraocular lens.
 17. The imaging lens unitclaim 14, configured for use as spectacles lens.
 18. The imaging lensunit claim 14, configured for use as an intracorneal lens.
 19. Animaging lens unit comprising an imaging lens having a lens region and anoptically transparent phase mask designed to provide at least threephase variation patterns to light component of at least three chromaticchannels passing through said imaging lens respectively, the opticallytransparent mask a height profile δd with respect to the lens regionthereby providing said at least three phase variation patterns, theheight profile δd being configured as follows $\begin{matrix}{{\delta \; {d(x)}} = {\frac{\lambda_{R}}{2{\pi \left( {n - 1} \right)}}\left( {{2\pi \; {N_{R}(x)}} + {\phi_{d}^{(R)}(x)}} \right)}} \\{= {\frac{\lambda_{G}}{2{\pi \left( {n - 1} \right)}}\left( {{2\pi \; {N_{G}(x)}} + {\phi_{d}^{(G)}(x)}} \right)}} \\{= {\frac{\lambda_{B}}{2{\pi \left( {n - 1} \right)}}\left( {{2\pi \; {N_{B}(x)}} + {\phi_{d}^{(B)}(x)}} \right)}}\end{matrix}$ wherein, λ_(R), λ_(G) and λ_(B) are wavelength defining atleast three chromatic channels, φ_(d) ^((R))(x), φ_(d) ^((G))(x), φ_(d)^((B))(x) are said phase variation patterns provided to light componentsof said at least three chromatic channels, n is a refractive index of amaterial of said optically transparent mask, N_(R), N_(G) and N_(B) areinteger numbers, and x is a position vector along said lens region; atleast one of said phase variation patterns being configured to provideimaging with extended depth of focus for the respective chromaticchannel.